2.1 IMMUNITY AND IMMUNIZATION
The immune system protects a host against pathogens by mounting an immune response which is specific to an antigen of an invading pathogen. The objective of immunization is to elicit an early protective immune response by administering to the host an attenuated pathogen, or an antigen associated with a pathogen. This approach has been implemented successfully to prevent a variety of infectious diseases, such as polio, tetanus and diphtheria.
Immunization may be accomplished passively by administering either preformed immunoreactive serum or cells; or actively by presenting a suitable antigenic stimulus to the host's immune system.
Passive immunization is useful for a host who cannot produce antibodies, or for those who might develop disease before active immunization could stimulate antibody production. However, antibodies produced following some infections, particularly those due to mycobacteria, fungi, and many viruses, are not effective in protecting against the infection. Rather, the action of lymphocytes and macrophages largely determines recovery from these diseases.
Active immunization may be achieved with either viable or non-viable antigenic agents. Viable agents are generally preferred because the immune response provoked is more reliable and long-lived. However, viable vaccines may cause serious illness in an immunologically incompetent host, such as patients receiving corticosteroids, alkylating drugs, radiation or immunosuppressants. The use of attenuated strains always carries the risk that the attenuated agent may recombine with host DNA and mutate into a virulent strain. See generally, Ada, G. L., 1989, Chapter 36, in Fundamental Immunology, 2nd edition, ed. Paul W. E., Raven Press, New York, pp. 985-1032; Cohen, S. N., 1987, Chapter 37, in Basic and Clinical Immunology, 6th edition, ed. Stites, Stobo and Wells, Appleton and Lange, pp. 669-689.